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京 都 大 学 大 学 院 理 学 研 究 科

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要旨集

２０１５年１月２１日（水）物 理 学 第 一 分 野

物理学第一分野ＤＣ３回生研究発表会
場所：理学研究科５号館 ５階・第四講義室
発表：２０分（別に質問１０分程度）
２０１５年１月２１日（水）９：００～

開始

目

次

１．Electronic Properties in heterostructures of strongly correlated electron systems
上田克（ ９：００）・・・・・

１

２．Replica symmetry breaking in trajectories of a driven Brownian particle
上田 仁彦（ ９：３０）・・・・・

２

３．Dynamical quantum effects in cluster dynamics of Fermi systems
尾崎 順一（１０：００）・・・・・

３

１０：３０～１０：４０

休憩

４．Statistical mechanics for athermal fluctuation: Non-Gaussian noise in physics
金澤輝代士（１０：４０）・・・・・

４

５．Classical Reduction of Quantum Master Equations as Similarity Transformation
紙谷 典和（１１：１０）・・・・・

５

６．Ferromagnetic critical behavior and critical universality
in itinerant-electron metamagnet UCoAl
軽部皓介（１１：４０）・・・・・

６

７．Inelastic X-ray Scattering Study of Plasmons in Liquid Alkali Metals
木村 耕治（１３：１０）・・・・・

７

８．Topological crystalline phases with order-two point group symmetry
塩崎
謙（１３：４０）・・・・・

８

９．Theoretical Study of Electron Dynamics in Multi-Orbital Antiferromagnetic Metals
杉本 高大（１４：１０）・・・・・

９

１２：１０～１３：１０

昼休み

１０．Numerical simulation of two-frequency forced Faraday waves with level-set method
髙木健太郎（１４：４０）・・・・・１０

１５：１０～１５：２０

休憩

１１．Correlation effects on a two-dimensional topological insulator
竹中 裕斗（１５：２０）・・・・・１１

１２．Two types of spinning motion of active deformable particles
多羅間充輔（１５：５０）・・・・・１２
１３．Phenomenological structure for large deviation principle in time-series statistics
根本 孝裕（１６：２０）・・・・・１３
１４．Excitonic finestructure and nonequilibrium phase transitionof the electron-hole system in diamond挾間 優治（１６：５０）・・・・・１４

１７：２０～１７：３０

休憩

１５．Angular dependence of high-field low-temperature phasein a Pauli limited d-wave superconductor
細谷健一（１７：３０）・・・・・１５１６．Nonlinear spin dynamics induced by intense terahertz magnetic field向井佑（１８：００）・・・・・１６
１７．Classifying neuronal spike trains as analog or digital coding
望月 泰博（１８：３０）・・・・・１７
１８．Quantum Gas Microscope of Two-electron Atoms‐high-resolution imaging of ultracold ytterbium atoms in a 2D optical lattice‐山本 隆太（１９：００）・・・・・１８

Electronic Properties in heterostructures of strongly correlated
electron systems
Condensed matter theory group

Suguru Ueda

Abstract We investigate interface physics of various artificial heterostructures involving Mott insulators.
Employing dynamical and static mean-field approximations, we uncover the presence of electronic states
specific to the heterointerface. We also elucidate the spatial modulation of such phases, and shed light oncorrelation effects in inhomogeneous systems.
© 2015 Department of Physics, Kyoto University

Remarkable experimental progress in fabricating atomically smooth interface allows us to access a wide range ofheterostructures based on strongly correlated electron systems. Since Ohtomo et al. found the conducting state at
the interface between insulators LaTiO3/SrTiO3 [1], many experimental and theoretical efforts have been put
toward the study of their rich, complex and potentially useful behavior.
Stimulated by them, we investigate the interface physics of such correlated heterostructures involving Mott
insulators with particular emphasis on the electronic/magnetic properties as well as electron correlation effects.
A fundamental issue addressed here is how electronic properties at the heterointerface differ from those of thebulk. Motivated by recent experiments on LaTiO3/SrTiO3, we first simulate the heterostructure of a Mott insulator(MI) and a band insulator (BI) [2]. The ground state phase diagram in the Hartree-Fock approximation reveals thepresence of electric/magnetic ordered phases that can emerge only at the interface layer. We elucidate that thesephases are derived from the strong spin/charge coupling around the interface.
It is known that the conducting interface between insulators is also observed at heterostructures of topologicalband-insulators (TI), whose interface (edge) state is protected by the non-trivial bulk band-structure. In order tostudy effects of electron correlation on such topological edge mode, we next focus on the proximity effect at the
MI/TI interface employing inhomogeneous dynamical mean-field theory (DMFT) with an exact diagonalization
method [3]. Our result shows that the topological edge state penetrates into the MI side, and induces strongly
renormalized mid-gap states within the Mott-Hubbard gap, which are robustly stable even in the presence of thestrong Coulomb interaction.
It is also demonstrated how correlation effects couple to the structure of the multilayered system. Taking an
example of LaVO3/SrVO3 [4], we investigate the MI/metal superlattice structure. Our DMFT results reveal thatthe electrical resistivity shows a peak structure as a function of temperature, and its peak position strongly
depends on the superlattice periodicity [5]. Despite its simplicity, our model provides a reasonable description forsome experimental findings in the LaVO3/SrVO3 superlattice: metal-insulator transition and the temperaturedependence of the resistivity.

References[1] A. Ohtomo, D. A. Muller, J. L. Grazul, H. Y. Hwang, Nature 419, 378 (2002).
[2] S. Ueda, N. Kawakami, and M. Sigrist, Phys. Rev. B 85, 235112 (2012).
[3] S. Ueda, N. Kawakami, and M. Sigrist, Phys. Rev. B 87, 161108 (2013).
[4] A. David, et al., Appl. Phys. Lett. 98, 212106 (2011).
[5] S. Ueda, N. Kawakami, (submitted).

Replica symmetry breaking in trajectories of a driven Brownian
particle
Nonlinear Dynamics Group

Masahiko Ueda

Abstract We study a Brownian particle passively driven by a velocity field obeying the noisy Burgers
equation. We numerically and theoretically demonstrate that the system exhibits replica symmetry
breaking in the path ensemble.
© 2015 Department of Physics, Kyoto University

It has been known that equilibrium states of mean-field spin glass models freeze into some irregular patterns at
low temperatures. The low temperature phase is characterized by a concept of replica symmetry breaking (RSB)[1]. RSB is a phenomenon reflecting localization of the state in a rugged free energy landscape and is detected bya physical quantity called overlap, which expresses how much two independent identical systems (replicas) aresimilar to each other. In RSB phase, an overlap takes a non-trivial value. These systems are one example which
exhibits stable and diverse states.
In nature, there are many systems which exhibit stable and diverse dynamics. A typical example of such
dynamical behavior is cell differentiation. In these systems, an observed time series is stable against perturbation
like limit cycle, while a variety of trajectories are observed like chaos and Brownian motion. Behaviors of thesesystems are characterized by a rugged probability landscape of the trajectory. A natural question is whether there
are any dynamical behaviors in which the concept of RSB in path probability appears.
Here, we present a non-equilibrium phenomenon where RSB plays an important role [2]. Concretely, we
consider a passive Brownian particle driven by a random velocity field, and study whether RSB in path ensemble
occurs when we focus on trajectories of the particle. As a result, we numerically find that, in case of a velocity
field driven by the noisy Burgers equation [3], an overlap between two independent particles becomes non-zero.
Furthermore, by changing a boundary condition of the model, we theoretically prove that the model can bemapped exactly to directed polymers in random potentials [4], which exhibit RSB. In addition, by comparing themodified system with the original system numerically, we conclude that the original system also exhibits RSB in
the path ensemble. This result provides one example where the concept of RSB is useful even in non-equilibrium
situation.

x

2.5

5120

2

5080

1.5

P(q)

5040
5000

1

0.5

t

4960
0

500 1000 1500 2000

q

0
0

0.2

0.4

0.6

0.8

1

Fig. 1. (Left) Trajectories of ten Brownian particles driven by a velocity field obeying the noisy Burgers
equation. (Right) Distribution of overlap for various time.

References[1] M. Mezard, G. Parisi, and M. A. Virasoro, Spin glass theory and beyond (World Scientific, Singapore, 1987).
[2] M. Ueda and S.-i. Sasa, arXiv:1411.1816.
[3] M. Kardar, G. Parisi, and Y.-C. Zhang, Phys. Rev. Lett. 56, 889 (1986).
[4] T. Halpin-Healy and Y.-C. Zhang, Physics Reports 254, 215 (1995).

Dynamical quantum effects
in cluster dynamics of Fermi systems
Condensed matter theory group,

Jun’ichi Ozaki

Abstract We study cluster dynamics of Fermi systems to clarify dynamical quantum effects in thosesystems. We simulate collision dynamics and drag dynamics in one dimension using time-dependent
density matrix renormalization group, and compare the obtained results with the semiclassical calculation.
The simplified model are suggested to explain the results.
© 2015 Department of Physics, Kyoto University

Recently non-equilibrium dynamics of cold atom systems has been enthusiastically targeted, because cold atom
systems are ideal as isolated quantum systems configured in laboratory, whose parameters can be modified
dynamically [1]. For example, the dynamics of quantum quench has been explored by suddenly changing the trap
potential and the interaction. Also, dynamics induced by a gradual change of parameters in real time has a lot
more to be investigated.
We study (i) collision dynamics of two fermion clusters and (ii) typical drag dynamics of several fermions in afermion cloud in one-dimensional continuous systems, with particular emphasis on the non-trivial quantum
many-body effects in systems whose parameters change (i) suddenly or (ii) gradually in real time. We adopt the
Fermi-Hubbard model and the time-dependent density matrix renormalization group method (t-DMRG) [2] tocalculate the dynamics of those systems, and we explain the simulation results by using our concise models.
Finally we clarify the origin of the non-trivial quantum effects in dynamics, by comparing the results and models
with semi-classical or mean-field treatments.
(i) Collision dynamics of two fermion clusters with contact interaction is simulated [3]. We calculate reflectanceand transmittance of the clusters. We elucidate that the quantum effects become extremely strong with theinteraction strength, leading to the transmittance much more enhanced than expected from semiclassicalapproximation. We propose a concise model as an application of the Bethe ansatz, which unveils the origin of thequantum effects and also explains the overall properties of the simulation results clearly. This model provides anintuitive perspective of the collision dynamics with contact interaction. Some potential applications, such as
repeated collisions, are addressed.
(ii) Typical drag dynamics of several fermionsin a fermion cloud in one-dimensional continuous
systems is simulated [4]. We calculate the dragforce on a trapped fermion cluster in a cloud of
another fermion species with contact interaction.
A non-trivial peak (Fig. 1) in the resistance force
is observed in the high cloud density region, and
it is clarified that the peak cannot be explained bya naïve mean-field theory, although the otherstructures are explained. This implies that someinternal degrees of freedom have a crucial role inFig. 1. Drag force versus cloud density
the excitation process. We also propose a criterion ofeffective ways in diffusive transport in a fermion cloud.
References[1] A. Sommer, M. Ku, G. Roati and M.W. Zwierlein, Nature 472, 201 (2011).
[2] S. R. White and A. E. Feiguin, Phys. Rev. Lett. 93, 076401 (2004).
[3] J. Ozaki, M. Tezuka and N. Kawakami, Phys. Rev. A 86, 033621 (2012).
[4] J. Ozaki, M. Tezuka and N. Kawakami, arXiv: 1402.6486.

Statistical mechanics for athermal fluctuation:
Non-Gaussian noise in physics
Advanced Statistical Dynamics Lab.

Kiyoshi Kanazawa

Abstract We derive Langevin-like equations with non-Gaussian noise for a wide class of athermal
systems from microscopic dynamics by developing an asymptotic method. We also show that the
non-Gaussian properties of the athermal noise are useful in inferring microscopic information of athermalbaths. Furthermore, we derive the full-order asymptotic formula for the steady distribution function for an
arbitrary non-linear friction, and show the high-order terms correspond to multiple-kicks during
relaxation. As a demonstration, we study the granular motors under viscous or dry friction. Our study
shows that our non-Gaussian model is the minimal model satisfying both universality and analytical
simplicity.
© 2015 Department of Physics, Kyoto University

Recently, experimental development has enabled us to investigate detailed properties of small
fluctuating systems, and studies on thermal fluctuation have attracted wide interest. To
understand these systems, the Langevin model has been used to describe dynamical properties ofthermal fluctuation. The Langevin equation is a linear stochastic model with the Gaussian noise,
and is analytically solvable. Moreover, this model can be asymptotically derived for a wide classof systems associated with single stochastic environments in the small noise limit[1], which
proves the universality of this model.
On the other hand, non-Gaussian fluctuation is reported to appear for athermally fluctuating
systems. A natural question would then arises: When and how does the non-Gaussianity emerge frommicroscopic dynamics? This question has not yet been fully answered. Indeed, the conventionalcoarse-graining theories fail to explain non-Gaussian behaviors at leading order because of thecentral limit theorem (CLT). In this presentation, we show an answer to this question. We
asymptotically derive Langevin-like non-Gaussian models for a wide class of stochastic systems
associated with thermal and athermal baths under three assumptions: (i)large system size,(ii)strong thermal friction, (iii)coexistence of the thermal and athermal noise[2, 3]. We also
show that the non-Gaussian properties of the athermal fluctuation can be used to infer microscopic
information of the athermal bath. Furthermore, we show the full-order asymptotic formula forthe steady distribution for an arbitrary non-linear friction, and illustrate that the high-orderterms directly correspond to multiple-kicks processes during relaxation by introducing a
diagrammatic representation[3]. As a demonstration, we study granular motors under viscous or
dry friction, and verify our formulation numerically. Our study shows that the non-Gaussian
Langevin equation is the minimal model satisfying both physical universality and analytical
simplicity.
References[1] N.G. van Kampen, Stochastic Processes in Physics and Chemistry, North-Holland (2007).
[2] K. Kanazawa, T.G. Sano, T. Sagawa, and H. Hayakawa, arXiv: 1407.5267 (2014).
[3] K. Kanazawa, T.G. Sano, T. Sagawa, and H. Hayakawa, arXiv: 1412.2233 (2014).

Classical Reduction of Quantum Master Equations as
Similarity Transformation
Non-equilibrium Physics Group

Norikazu Kamiya

Abstract We investigate classical reduction of quantum master equations by similarity transformation of
the Liouvillian. The method is applied to quantum energy transport and the transport efficiency problem.
© 2015 Department of Physics, Kyoto University

Transport efficiency (ζ)

The quantum master equation approach is a standard tool in studies of quantum open systems.
In their analysis, the rotating wave approximation (RWA) is commonly used on the quantum masterequation to obtain the Lindblad equation, which ensures the trace-preserving property andcomplete positivity. In the RWA, the diagonal and off-diagonal elements of the density matrix
in the energy representation are split, resulting in the diagonal stationary solution. However,internal energy currents are off-diagonal in the energy basis. Consequently, there is no resulting
energy flow for the RWA master equation [1].
The RWA can be regarded as classical reduction of the quantum master equation. These classicalreductions are formally described by the projection operator technique. However, the projection
operator technique does not give a solution of the problem of energy currents in the RWA.
In this work, we present a new formulation of quantum-to-classical reduction of quantum masterequations utilizing a similarity transformation of the Liouvillian [2]. Our formulation reveals
the necessity of the transformation of observables, which gives a solution of the problem in
the RWA scheme. We discuss the transformation of an energy current using a certain simple example.
Moreover, we show that the quantum
0.6
master equation can be exactly replaced withclassical master equation for the
0.5
calculation of a quantity such as quantum
transport efficiency. We discuss the single
0.4
exciton on a tight binding Hamiltonian with
0.3
dissipation. The transport efficiency isdefined as how often the particle trapped at
0.2
the trapping site. We confirm the exact
quantum-to-classical replacement by the0.1
numerical calculation (Fig. 1).
0
0

5

10

15

20
Time (t)

25

30

35

40

Fig. 1: Transport efficiency as a function of time. Dashedline denotes the quantum dynamics and solid-line denotes
the replaced classical dynamics.

References[1] H. Wichterich, M. J. Henrich, H-P. Breuer, J. Gemmer, and M. Michel, Phys. Rev. E 76, 031115 (2007)[2] N. Kamiya, submitted.

Ferromagnetic critical behavior and critical universality
in itinerant-electron metamagnet UCoAl
Quantum Materials Lab.

Kosuke KARUBE

Abstract In order to investigate static and dynamic properties around a critical endpoint (CEP) and atricritical point (TCP) in itinerant ferromagnetic systems, we have performed nuclear magnetic resonanceand nuclear quadrupole resonance measurements for an itinerant metamagnet UCoAl. We observed thedevelopment of magnetic fluctuations near the CEP and TCP.
© 2015 Department of Physics, Kyoto University

Itinerant-electron ferromagnetic (FM) materials are expected to follow an universal 3D (temperature - magnetic
field - pressure) phase diagram characterized by the presence of a tricritical point (TCP), where a FM transitionchanges from second-order to first-order, and a critical endpoint (CEP), where a first-order field-induced FM
transition [metamagnetic (MM) transition] changes to a crossover[1]. We focused on an itinerant metamagnet
UCoAl as a suitable material to clarify the static and dynamic magnetic properties around these critical points
since it shows a MM transition at low fields. We have performed 27Al- and 59Co-nuclear magnetic resonance(NMR) and nuclear quadrupole resonance (NQR) measurements in order to estimate the c-axis magnetization (Mc)(order parameter) and the c-axis magnetic fluctuations (Sc) under the precise control of tuning parameters:
temperature (T), magnetic field along the c-axis (Hc), uniaxial pressure along the b- and c-axes (P||b,c), andsubstituted Fe concentration (x).
At ambient pressure, we observed the Ising-type anisotropy along the c-axis in both magnetization and magnetic
fluctuations. In addition, Sc exhibits a broad maximum at Tmax ~ 20 K even in Hc = 0, which is a characteristicfeature in itinerant metamagnets. The contour plots of Mc and Sc on the T - Hc phase diagram at ambient pressure
show the presence of the CEP at (T, 0Hc) ~ (12 K, 1.0 T) and the divergence of Sc at the CEP as shown in Fig. 1.
From the estimation of critical exponents around the CEP, wefound that the MM critical behaviors in UCoAl are categorized
into a 3D-Ising universality class as observed in the gas-liquidand 3D-Mott transitions[2]. Uniaxial pressure P||b and P||c giverise to the opposite magnetic response; P||b suppresses the FM
state, whereas, P||c induces the FM state even in Hc = 0.
Furthermore, the peak intensity of Sc at Tmax ~ 20 K in Hc = 0 is
suppressed by P||b, whereas, it is enhanced by P||c andmaximized around (T, P||c) ~ (20 K, 0.16 GPa), indicating thatFM fluctuations are enhanced toward the TCP[3]. Fe-substituted
U(Co1-xFex)Al clearly shows the first-order FM transition in Hc= 0 in the x range of 1-2%. The first-order FM transition line
and Tmax line seem to merge around (T, x) ~ (20 K, 2.5%), whichsuggests the presence of the TCP as theoretically predicted.
Taking the above P||c measurement into consideration, Sc at Tmax
~ 20 K develops with approaching the TCP.
In conclusion, we have confirmed the presence of the universal
3D phase diagram in UCoAl and revealed the development of
FM critical fluctuations at the CEP and TCP.
References
Fig. 1. Contour plots of (a) c-axis[1] D. Belitz et al., Phys. Rev. Lett. 94, 247205 (2005).
magnetization (Mc) and (b) c-axis magnetic[2] K. Karube et al., Phys. Rev. B 86, 024428 (2012).
fluctuations (Sc) on the T - Hc phase[3] K. Karube et al., J. Phys. Soc. Jpn. 83, 084706 (2014).
diagram at ambient pressure in UCoAl.

Inelastic X-ray Scattering Study of Plasmonsin Liquid Alkali Metals
Physics of Disordered Systems GroupKoji Kimura

Abstract We investigate plasmons in liquid alkali metals using inelastic x-ray scattering (IXS) techniques.
Variations in the plasmon behaviors upon melting, observed in the IXS experiments, are discussed in termsof structural disorder in liquid metals. In particular, the change in the line width is reproduced by a newly
derived formula.
© 2015 Department of Physics, Kyoto Universityliqsol
DE 1/2( q) - D E 1/2( q) [eV]

D E 1/2(q) [eV]ℏ𝜔p 𝑞 [eV]The electronic states in liquid metals are less clearly understood than in crystalline systems, because of thedisordered structure in the liquid state. Several properties of liquid metals, such as optical reflectivity [1] andelectronic excitations [2], have not been sufficiently studied in relation to the disordered structure. In fact, effectsof ions on these properties in liquid metals have been discussed only based on the similarity of the local structurein the liquid state to that in the solid state, and influences of the structural disorder have not been clarified adequately.
Behaviors of plasmon, a collective excitation of electrons, reflect(a)
3.6
the electron-ion interaction. Thus we can investigate the influence of
the ions on the electronic excitations in liquid metals, through theplasmons.
In this work, we performed inelastic x-ray scattering (IXS)
3.5
experiments for solid and liquid Rb to clarify the effect of structural
disorder on plasmon behaviors in liquid metals [3]. We observed
inelastic peaks arising from plasmons in the IXS spectra for solid andliquid Rb. From the IXS spectra, we determined the plasmon
3.4
excitation energy (ℏ𝜔p) and the line width (Δ𝐸1/2 ) as a function of
the momentum transfer q.
(b)Figure 1 (a) shows ℏ𝜔p (𝑞) of solid and liquid Rb. As seen in
0.5
the figure, ℏ𝜔p (𝑞) of liquid Rb tends to increase with q, while thatof solid Rb shows cusp-like shape, where ℏ𝜔p (𝑞) increases in q <20
0.5 Å-1 and decreases in q > 0.6 Å-1 with increasing q. Figure 1 (b)shows Δ𝐸1/2 (𝑞) of solid and liquid Rb. We can see that Δ𝐸1/2 (𝑞)0.2 0.4 0.6 0.8
of liquid Rb tends to be narrower than that of solid Rb for q < 0.5 Å-1
q [Å ]
1. This tendency is more clearly seen in the inset of Fig. 1 (b), which1
shows the difference of Δ𝐸1/2 (𝑞) between solid and liquid Rb. The
variations in ℏ𝜔p (𝑞) and Δ𝐸1/2 (𝑞) upon melting are discussed insolid Rb
terms of the difference of the ionic structures between solid Rb andliquid Rb
liquid Rb, and it is suggested that the effect of the ionic potential on
0
0.2
0.4
0.6
0.8
the plasmon behavior in liquid Rb is weaker than that in solid Rb. In-1q [Å ]
particular, we have newly derived a formula for evaluating Fig. 1. (a) Plasmon excitation energy, ℏ𝜔p ,Δ𝐸1/2 (𝑞) in liquid metals at q = 0 [4], where disordered structure in and (b) the line width, Δ𝐸1/2 , in solid andthe liquid state is taken into account, and reproduced the liquid Rb as a function of q. The inset shows
the difference of Δ𝐸1/2 between solid andliq
sol
experimentally observed narrowing of the line width.
liquid Rb, Δ𝐸1/2 (𝑞) − Δ𝐸1/2(𝑞).
References[1] B. Knuth, F. Hensel, and W. W. Warren Jr, J. Phys.: Condens. Matter 9 2693 (1997).
[2] J. P. Hill, C. C. Kao, W. A. C. Caliebe, D. Gibbs, B. Hastings, Phys. Rev. Lett. 77, 3665 (1996).
[3] K. Kimura, K. Matsuda, N. Hiraoka, et al., Phys. Rev. B 89, 014206 (2014).
[4] K. Kimura, K. Matsuda, and M. Yao, J. Phys. Soc. Jpn. 82, 115001 (2013).

Topological crystalline phases with order-two point groupsymmetry
Condensed matter theory groupKen Shiozaki

Abstract We present the complete classification of topological crystalline insulators and superconductors
with order-two point group symmetry by using K-theory. The obtained classification shows that defectgapless states can be considered as boundary states, and a novel realization of the bulk-boundary
correspondence in terms of K-theory.
© 2015 Department of Physics, Kyoto University

In this decade, topological phases have provided a new area of study in condensed matter physics. Topologicalinsulators and superconductors are classes of topological phases of free fermions. The bulk materials are describedby the non-interacting single particle Hamiltonians. The bulk shows no low-energy excitation because of a finite
excitation gap, thus, all physically observable phenomena can occur at their boundaries or topological defects. The
topological nontriviality of the bulk reflects an existence of robust gapless excitations at the boundaries or theirdefects. Topological phases are classified according to symmetries such as the time-reversal symmetry and theparticle-hole symmetry. Space group symmetries originating from crystalline structures also give finerclassifications of topological phases, which enable some trivial phases without crystalline symmetries to be
topologically nontrivial phases. It is naively expected that the gapless boundary modes in topological crystalline
insulators and superconductors are fragile against disorders because these specific symmetries are microscopicallysensitive to small disorder effect. But recent studies of topological crystalline insulators have shown that if thesymmetries are preserved on average, then for some symmetry classes the existence of gapless boundary states israther robust. Moreover, surface gapless states protected by the mirror reflection symmetry have been observed
experimentally. Such recent developments motivate us to classify topological crystalline insulators andsuperconductors, and reveal their unified low-energy physics. Some topological crystalline phases with simple
point group symmetries have been classified. However, there is no knowledge of an algorithmic method of theclassification of topological crystalline phases for general space group symmetries.
For order-two point group and magnetic point group symmetries, we can successfully classify all the topologicalphases of free fermions by using the Clifford algebras and K-theory. The order-two point group symmetries weconsider are general and include Z2 global spin flip, reflection, twofold rotation, inversion, and their magnetic
symmetries. An additional order-two symmetry provides a new generator of the Clifford algebra and shifts
classifying spaces of Hamiltonians. We can construct various K-group isomorphisms between different spacedimensions and symmetries, leading to the complete classification of the topological crystalline phases withorder-two point group symmetries. Various symmetry protected topological phases and gapless modes will beidentified and discussed in a unified framework. Obtained topological classification suggests that defect gaplessexcitations can be considered as boundary states of lower-dimensional crystalline insulators and superconductors.
We also present topological classification of symmetry protected Fermi points. The bulk classification and thesurface Fermi point classification provide a novel realization of the bulk-boundary correspondence in terms of
K-theory.

Reference
Ken Shiozaki and Masatoshi Sato, Phys. Rev. B 90, 165114 (2014).

Theoretical Study of Electron Dynamics in Multi-Orbital
Antiferromagnetic Metals
Physics of Matter: Condensed Matter Physics (YITP)

Koudai Sugimoto

Abstract In order to understand the multi-orbital physics in antiferromagnetic phase, we investigate ironarsenide and chromium. We elucidate the origin of in-plane anisotropic resistivity in iron arsenide with amemory-function approach, and calculate resonant inelastic x-ray scattering spectra in chromium.
© 2015 Department of Physics, Kyoto University

Chromium (Cr) is known to be a typical antiferromagnetic metal characterized by an incommensurate
spin-density-wave (SDW) ground state due to the Fermi-surface nesting. The magnetic and electronic properties
of Cr have widely been investigated for a long time. Such studies of 3d transition-metal compounds have beenrenewed by a recent discovery of iron-based superconductors, and newly focused on the aspect of the multi-orbital
nature of 3d atoms. In the field of iron-based superconductors, it is now recognized that the orbital degrees of
freedom play a crucial role not only in the mechanism of superconductivity but also in the understanding of
antiferromagnetic metallic phase adjacent to the superconducting phase. Therefore, it is important to include all of$3d$ orbitals in theoretical works of transition-metal compounds. Physical properties of the typicaltransition-metal compound, chromium, must be accordingly reinvestigated from this viewpoint. In this thesis, wetheoretically study the electron dynamics in multi-orbital antiferromagnetic metals, especially focusing on ironarsenide and Cr. Our aim of studying both materials is to deepen our understanding of the physics ofantiferromagnetic transition-metal compounds characterized by multi-orbital nature.
The parent compound of iron-arsenide superconductors exhibits an antiferromagnetism with stripe-typeorder below Neel temperature. The band structure of the stripe phase is characterized by Dirac-type dispersions,which are induced by the folded paramagnetic dispersion with multi-orbital nature. In the stripe phase, theresistivity shows anisotropy in the conducting Fe-As plane. The experimental data suggest that impurities play a
crucial role in the anisotropy. We start with a five-3d-orbital Hubbard model, and use a mean-field approximation.
Then, we examine the anisotropy of resistivity by applying the memory-function approach treating isotropic
nonmagnetic impurities. The resistivity obtained by the memory-function approach yields a proper anisotropic
behavior near the undoped region: The resistivity in the antiferromagnetically ordered direction is first smaller
than that in the ferromagnetically ordered direction, and the anisotropy reverses as holes are introduced. The
origin of the anisotropy can be understood from the interplay of impurity scattering and the character of Fermi
surfaces including the Dirac-type one [1].
Motivated by the crucial role of orbitals in the iron-arsenide antiferromagnetic metallic phase, we
reinvestigate electron dynamics of the typical itinerant SDW metal, Cr [2]. We again use multi-band Hubbard
model composed of 3d and 4s orbitals. After the SDW mean-field approximation, we obtain the dynamical spin
susceptibilities and L3-edge resonant inelastic x-ray scattering (RIXS) spectra by employing random phaseapproximation. In our calculation, we assume the perfect commensurate SDW state. We find a collective
spin-wave excitation undamped up to ~0.6 eV. Above the energy, excitations overlap individual particle-holeexcitations as expected. In RIXS spectra, particle-hole excitations with various orbital channels show a large
spectral weight, masking the spectra of the spin collective mode. However, it may be possible to detect the
spin-wave excitation in RIXS experiments in the future if resolution is high enough.
The above two topics in this thesis provide the evidence of crucial roles of 3d orbitals in theantiferromagntic metal of 3d transition-metal compounds. This contributes to the deeper understanding of thephysics of multi-orbital antiferromagnetic metal.
References[1] K. Sugimoto, P. Prelovšek, E. Kaneshita, and T. Tohyama, Phys. Rev. B 90, 125157 (2014).
[2] K. Sugimoto, Z. Li, E. Kaneshita, K. Tsutsui, and T. Tohyama, Phys. Rev. B 87, 134418 (2013).

Numerical simulation of two-frequency forced Faraday waves withlevel-set method
Fluid Physics Group

Kentaro Takagi

Abstract We performed a numerical simulation of two-frequency forced Faraday waves with level-set
method. The pattern observed only in two-frequency forced Faraday waves was numerically reproduced
for the first time. Our numerical results show that assumption of a weakly nonlinear analysis of the pattern
c 2015 Department of Physics, Kyoto University
is incomplete.
⃝

a2 /g

Faraday waves are standing waves forming a fascinating pattern in the fluid interface subjected totypical vertical periodic force. Although Faraday waves were found two hundred years ago, many
researchers have been attracted even today since new nonlinear phenomena continue to be found.
For example, there are two-frequency forced Faraday waves. In some experiments [1], more complex
patterns than the single frequency case are reported. The linear stability analysis [2] and the weakly
nonlinear analysis [3] are applied to two-frequency Faraday waves were developed. While a numberof patterns of two-frequency forced Faraday waves can be described by these analyses, some essential
but complex patterns have not been understood. Therefore, three dimensional numerical simulationis required to elucidate the mechanism. In this study, we performed the first numerical simulation of
two-frequency forced Faraday waves.
Périnet et al. [4] performed the first three dimensional numer7
ical simulation of Faraday waves with single frequency vibration.
6
Their results are consistent with those of laboratory experiments
5quantitatively. However, their numerical scheme is not suitable for4
recent complex Faraday waves, because their implementation can
3
not describe complex interfaces with large deformation. Since we
2Linear analysis
1
aim to reproduce not only two-frequency forced Faraday waves, but
numerical results0
also other complex patterns, we adopt an interface tracking scheme
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
method called level-set method diﬀerent from Périent et al.
a1 /g
In order to validate our three dimensional simulation scheme, theFig.1 Comparison of numericalresults were compared with those of the linear stability analysis [2].
results with linear stability analyAs shown in Fig. 1, our results are consistent with those of thesis. The x and y axes represent thelinear stability analysis quantitatively. Moreover, for the validation
normalized
vibration amplitudes.
in a nonlinear regime, we reproduced square and hexagon patterns
with identical physical properties to the experiment [1]. After thesevalidations, we succeeded in reproducing the rhomboidal pattern
for the first time. This pattern is observed only in two-frequency
forced Faraday waves. The interface profile is shown in Fig. 2.
Finally, we check whether the assumptions made in the weakly
nonlinear analysis for the rhomboidal pattern [3] are reasonable bycomparing with our simulation results. It is assumed that a damping parameter are small and the amplitudes vary slowly. However,
our numerical results show that the assumptions are not reasonable.
Consequently, the weakly nonlinear analysis of the rhomboidal pattern is incomplete and has to be improved.

References[1] H.Arbell and J.Fineberg, Phys. Rev. Lett. 85:756–759 (2000).
[2] T.Besson, W.Edwards and L.Tuckerman Phys. Rev. E 54:507–513 (1996).
[3] J.Porter and M.Silber, Phys. Rev. Lett. 89:084501 (2002).
[4] N.Périnet, D.Juric and L.Tuckerman J. Fluid Mech. 635:1 (2009).

Fig.2 The surface profile ofrhomboidal patterns colored by
its interface height.

Correlation effects on a two-dimensional
topological insulator
Condensed matter theory group

Yuto Takenaka

Abstract We address the effect of Coulomb interactions in topological band insulators.
In order to study how interactions affect the topological properties, we apply the variational
Monte Carlo method to the two-dimensional topological model.
© 2015 Department of Physics, Kyoto University

Recently, the topological band insulator (TBI) has been one of the current important issues incondensed matter physics. In TBI, spin-orbit coupling leads to a time-reversal invariant band
structure with a full insulating gap in the bulk and gapless edge states on the boundaries. Whilethe essential features of the TBI can be described as a one-body problem, the electron correlation
effects are expected to create novel topological states. For example, iridium compounds, such as
Na2IrO3 and A2Ir2O7 (A=Pr, Eu) have recently been proposed as materials in which the interplay ofspin-orbit coupling and electron interaction effects might be important [1, 2].
Many theoretical analyses of electron correlation effects in TBIs are calculated by mean field
theory, such as dynamical mean field theory(DMFT) or slave-boson mean field theory (SVMF)
in the periodic boundary condition [2, 3]. In orderto study how electron interactions affect the edge
states and bulk states respectively, it is important
to consider spatial dependence due to the lack oftranslational symmetry perpendicular to theboundaries in finite dimensions.
In this study, we investigate a generalized
Bernevig-Hughes-Zhang
model
[4]
havingFig. 1. U-dependence of the electron distribution
electron correlations with the variational Monte
Carlo (VMC) method. With this method, we canaccurately estimate the ground-state properties. In order to study spatial dependence of Coulombinteractions, we consider the periodic (open) boundary condition in x- (y-) direction and introduce
spatially-dependent (y-dependent) variational parameters.
We calculate the ground-state energy and the double occupancy. From these data, we show that afirst-order transition occurs in this model, which is accompanied by the coexistence region by thehysteresis. In order to clarify the topological properties, we consider the electron distribution. We
show that the qualitative behavior of the electron distribution clearly changes around the Mott
transition point, and edge states disappear (Fig.1). In order to discuss how the correlations affectelectronic properties in the TBI, we also calculate the spin and charge structure factors. Comparing
these data with previous studies, we discuss the effect of Coulomb interactions in the TBIs.
References[1] A. Shitade, et al, Phys. Rev. Lett. 102, 256403 (2009)[2] D. A. Pesin and L. Balents, Nat. Phys. 6, 376 (2010)[3] T. Yoshida, S. Fujimoto, and N. Kawakami, Phys. Rev. B 85, 125113 (2012)[4] B. A. Bernevig, T. L. Hughes and S. C. Zhang, Science, 314, 1757 (2006)

Two types of spinning motion of active deformable particles
Nonlinear Dynamics Group

Mitsusuke Tarama

Abstract: Two types of spinning motions of active deformable particles are investigatedtheoretically. One is the spinning motion due to the rotation of the whole body. Besides, shapedeformation may travel along the interface, which is regarded as another type of spinning motion
in terms of the particle dynamics. © 2015 Department of Physics, Kyoto University
Active matter, or active particle, is an object that converts potential energy into kinetic energy in itself andconsequently undergoes systematic movements. Major realisations of active matter are found in colloidal systems
such as Janus colloids, where one side of the surface is functionalised to have a difference in, for instance, thechemical property from the other. Colloidal active particles are usually rigid, but there are also deformable active
particles like liquid droplets that undergo chemical reactions on the interface, which is soft and does change its
shape during its spontaneous movement. Indeed, deformability is of basic importance for most of the biologicalprocesses. Dynamics of such active deformable particles are actually what we are interested in here.
Active motions, which are the motions that active particles exhibit spontaneously, include not only atranslational motion of the centre of mass, but also a spinning motion, i.e. the rotation around the centre of mass,
deformations of the shape, a self-replication, and a fusion of active particles. In this presentation, we concentrateourselves on the spinning motion of active deformable particles. We propose the argument that there are at least
two types of spinning motion for active deformable particles. On the one hand, an active particle undergoes aspinning motion due to the rotation of a whole body (type-I spinning motion). This type of spinning motion
corresponds to that of a classical rigid body. On the other hand, when the particle is soft and hence does change its
shape, the deformation of the particle shape may travel along the interface. In terms of the dynamics of the particle,
this traveling wave of shape deformation is regarded as a spinning motion (type-II spinning motion).
In order to confirm this proposition, we demonstrated the dynamics of active deformable particles byintroducing model equations for each spinning motion. We described the rotation of the particle by anantisymmetric tensor variable, and the shape deformation by using symmetric deformation tensors. To keep theargument simple and general, we derived nonlinearly coupled time-evolution equations for these variables as wellas the velocity of the centre of mass from symmetry considerations, which therefore do not depend on any specificmechanisms. First, we showed that the type-I spinning motion appears when the antisymmetric tensor
representing the rotation of the particle takes a non-vanishing value by breaking the symmetry spontaneously[1,2,4]. In contrast, we analytically predicted and verified numerically [3] that the coupled equations of motion for
the quadratic and quartic deformations experience instability in the angular direction for finite magnitude of
deformations, and thus the deformations starts to travel along the interface. This traveling wave of deformations isregarded as a spinning motion concerning the dynamics of the particle. Since this spinning motion appears
independently of the rotational variable of the particle and instead the traveling wave of deformations plays animportant role, we may conclude it is a different dynamical mode from the type-I spinning motion and indeed it is
the type-II spinning motion. In this case, an oscillatory motion of the shape deformation was also obtained. We
address that the model for the type-II spinning motion is further generalised to the coupled equations of nth- and2nth-mode deformations, which possess mathematical symmetry for different n. Each of these two types ofspinning motion has been observed experimentally. Moreover, a cell that exhibits the type-II spinning motion was
found to experience an oscillation of the deformation as well.
References[1] M. Tarama and T. Ohta, J. Phys.: Condens. Matter 24, 464129 (2012).
[2] M. Tarama and T. Ohta, Prog. Theor. Exp. Phys. 2013, 013A01 (2013).
[3] M. Tarama and T. Ohta, Phys. Rev. E 87, 062912 (2013).
[4] M. Tarama, Y. Itino, A.M. Menzel, and T. Ohta, Eur. Phys. J. Special Topics 223, 121–139 (2014).

Phenomenological structure for large deviation principle intime-series statistics
Nonlinear Dynamics Group

Takahiro Nemoto

Abstract We propose a calculation method of the large deviation functions of time-averaged quantities
based on the idea for measuring thermodynamic functions. We then apply our method to non-equilibrium
lattice gasses and study the effective descriptions of the exponentially biased ensembles in them.
© 2015 Department of Physics, Kyoto University

Thermodynamic functions provide quantitative relations among macroscopic observables inequilibrium systems. These functions are constructed by operational (or phenomenological)
manners within the theory of thermodynamics. On the other side, from the statistical mechanics,
which connects the thermodynamic functions to microscopic theories such as Newtonian dynamics,
one has known that the thermodynamic functions also play a role of the large deviation functions ofthermodynamic variables. This structure is suggestive, and recently, several results, for extending
this structure to the large deviation functions of time-averaged quantities in non-equilibrium
systems, have been developed.
In time-series statistics, the large deviation functions are connected to the frequency of rare
events. Then, what follows from the phenomenology similar to thermodynamics for constructing
these large deviation functions within an operational manner, if it exists? On one hand, the large
deviation functions are phenomenologically obtained. On the other hand, it describes the frequencyof rare events characterized by large deviation principle. Such a phenomenology should become arare event sampling method that can be used in real experiments. In this talk, I am going topropose a computational method of large deviation functions of time-averaged quantity [1], which isbased on this idea.
We apply the proposed method to numerical studies of non-equilibrium lattice gases, which are
an ASEP (Asymmetric Simple Exclusion Process) and a KCM (Kinetically constrained model). Bothof them are many-body systems and several non-trivial results have been found in the previous
studies of them. We calculate the cumulant generating functions (or large deviation functions) of
time-averaged quantities in these models, from which we study the dominant interactions in thosemodels, when rare events are taking place (Fig.1).

Fig. 1. Cumulant generating functions of non-equilibrium lattice gasses. By combining our method withtruncated transition rates, we investigate which interactions are important in rare events. The left figure isfor ASEP with a one-body-potential approximation, and the right figure is for a KCM with ashort-range-interaction approximation.

References[1] T. Nemoto and S. Sasa, Phys. Rev. Lett. 112, 090602 (2014).

Excitonic finestructure and nonequilibrium phase transitionof the electron-hole system in diamond
Solid-state spectroscopy group

Yuji Hazama

Abstract We investigate nonequilibrium phase transition of the electron-hole system in diamond. We
elucidate the origin of the fine structure of excitons and observe a signature of a continuous transition
below the critical temperature. We find that the unconventional phase transition is caused by dissipationof carriers due to the recombination.
© 2015 Department of Physics, Kyoto University

Temperature

Photoexcited electron-hole systems in semiconductors have long been investigated because they are suited tothe study of nonequilibrium many-body physics. It is well known that the introduction of valley degrees of
freedom in indirect-gap semiconductors drastically changes macroscopic behaviors of the system. For example,
few-body bound states of electron-hole pairs (polyexcitons) are considered to be stabilized [1] in silicon. It wasalso expected that polyexcitons would strongly affect the gas-liquid phase transition between the exciton and theelectron-hole liquid (EHL). However, the relation between valley degrees of freedom and the stability ofpolyexcitons is unexplored because the small binding energies of polyexcitons in silicon hinder detailed
spectroscopic measurements. The influence of polyexcitons on the phase transition is also unknown because weneed to access temperatures sufficiently lower than the critical temperature of EHL though it is impossible forsilicon by using standard cryogenic techniques.
In order to overcome such difficulties, we focused on diamond whose polyexcitons’ binding energies andcritical temperature are extremely high: a few tens of meV [2] and 165 K [3], respectively. We first investigatedthe origin of the fine structure of excitons by high-resolution spectroscopy, and found that the fine structure occurs
due to the competition between three effects: the spin-orbit interaction of the hole, the exchange interaction
between the electron and the hole, and the effective mass anisotropy of the electron. Second, we proposed anddemonstrated a photoexcitation method by which the eﬀective temperature of excitons can be kept as low as 15 K
up to the density of 1019 cm-3. Finally, we performed photoluminescence spectroscopy at various temperatures anddensities using our new excitation method. We observed polyexcitons below 30 K while thermodynamic theory
predicts that EHL is more stabilized than polyexcitons. We also found that the gas-liquid phase transition below
25 K shows the signature of a continuous phase transition while a gas-liquid phase transition below criticaltemperature is of first-order in general (see Fig. 1). We revealed
Critical pointthat the unprecedented transition is caused by the nonequilibrium
165 K
nature of the electron-hole system originating from dissipation ofcarriers due to the recombination.
LiquidGas
Our findings are important in two senses. First, the origin of
Gas & Liquid
25 K
the excitonic fine structure will benefit the investigation of theinternal structure of polyexcitons. Second, our result on thenonequilibrium gas-liquid phase transition indicated that
New statedissipation can drastically change thermodynamic properties of asystem even for the most elementary phase transition.
Density
References[1] J. S. Wang and C. Kittel, Phys. Lett. 42A, 189 (1972).
[2] J. Omachi et al., Phys. Rev. Lett. 111, 026402 (2013).
[3] R. Shimano et al., Phys. Rev. Lett. 88, 057404 (2002).

Fig. 1: Phase diagram of electrons and holes
in diamond. Dotted area is occupied by thecoexistent phase in equilibrium. A
nonequilibrium continuous phase transition
was found in the region labeled by “newstate”.

Angular dependence of high-field low-temperature phasein a Pauli limited d-wave superconductor
Condensed matter theory group

Kenichi Hosoya

Abstract We investigate the angular dependence of the high-field low-temperature (HFLT) phase in a heavy
fermion superconductor CeCoIn5 on the basis of a microscopic approach. We confirm the picture that, withincreasing the tilt angle, both the antiferromagnetic (AFM) and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) orders
are gradually suppressed, and that disappearance of the AFM order down to the zero-temperature limit occurs at alower angle than that of the FFLO state.
© 2015 Department of Physics, Kyoto University

Recently, the quasi-two-dimensional (Q2D) heavy-fermion superconductor CeCoIn5 has attracted much attention
due to a possible realization of a FFLO superconducting (SC) state in its high-field low-temperature (HFLT) SC
phase [1]. On the other hand, neutron scattering measurements in H ⊥ c have revealed the existence of an
incommensurate antiferromagnetic (AFM) order within the HFLT SC phase [2]. To try to understand the origin of
this AFM order, several mechanisms have been proposed. We have proposed that AFM order is induced by thed-wave pairing symmetry and a strong Pauli-paramagnetic pair-breaking (PPB) effect [3].
More recently, NMR data obtained by tilting the field direction from the a-b plane have revealed that with tilting
the field from the a-b plane, the AFM ordered region is pushed down to lower fields and pure (nonmagnetic)
FFLO phase appears just below Hc2 [4].
In this study, we discuss the angular dependence of the HFLT phase. We confirm that the appearance of pure
FFLO state observed in the NMR measurement is due to the incomplete nesting condition. In this calculation, we
find that, with increasing the tilt angle, both the AFM and FFLO orders are gradually suppressed, and thatdisappearance of the AFM order in the zero-temperature limit occurs at a lower angle than that of the FFLO state.
We also find that, in agreement with the observation [4], the AFM order in real space tends, as the field direction
is tilted, to occur only close to the FFLO nodal planes [5].

Fig. 1. H-T phase diagrams in the H ⊥ c case and at the rotated angle.

References[1] H. Adachi and R. Ikeda, Phys. Rev. B 68, 184510 (2003).
[2] M. Kenzelman et. al., Science. 321, 1652 (2008) .
[3] Y. Hatakeyama and R. Ikeda, Phys. Rev. B 83, 224518 (2011).
[4] K. Kumagai, H. Shishido, T. Shibauchi, and Y. Matsuda, "Angle Dependence of Magnetic Order and the
FFLO Phase of CeCoIn5" preprint (unpublished).
[5] K. Hosoya and R. Ikeda, Phys. Rev. B 88, 094513 (2013).

Nonlinear spin dynamics induced byintense terahertz magnetic field
Solid State Spectroscopy GroupYu Mukai

Abstract Nonlinear spin dynamics in the antiferromagnet crystal has been investigated by time-resolved
magneto-optical measurements with an intense terahertz magnetic field excitation. We found that thefrequency of the antiferromagnetic resonance is red-shifted and the decay rate of the magnetization
change is increased under the strong terahertz magnetic field.
© 2015 Department of Physics, Kyoto University[1] T. Kampfrath et al., Nature Photon. 7, 680 (2013).
[2] Y. Mukai et al., Appl. Phys. Lett. 105, 022410 (2014).

Frequency(THz)
Normalized
amplitude

Figure 1(a) shows the observed MO signal
corresponding to the magnetization change associatedwith an antiferromagnetic resonance. We applied theanalytic signal approach to obtain the instantaneous
frequency and envelope amplitudes to elucidate thedynamical feature of the signal. Figure 1 (b) shows thetime-responses of the instantaneous frequency obtained
from temporal waveforms at different excitation
intensities. One can see clearly large frequency shift
occurs at the high intensity case. The redshift of theantiferromagnetic resonance of HoFeO3 depends on theamplitude of the magnetization oscillation. This result
provides a direct proof of the nonlinearity in thecollective motion of the antiferromagnetic spins. In
addition, we found that the decay process of themagnetization change has strong excitation intensity
dependence. As shown in the Fig. 1(c), the decay time of
the magnetization amplitude becomes short for the highintensity excitation. This suggests that a nonlinear spin
relaxation process should take place in the strongly
driven regime.
References

Magnetization
change (a. u.)

Nonlinear dynamics of many-body spin systems has been an important research field and attracted considerable
attention from the perspective of fundamental physics and technological applications. Recently a considerable
research interest has been focused on spin dynamics of antiferromagnet whose magnetic response takes place inthe terahertz (THz) frequency region due to a large exchange interaction between spins. THz magnetic fieldresonant to the antiferromagnetic mode is the most advantageous for the direct magnetic excitation suppressing
undesirable thermal effects [1]. However, no observation of the nonlinear feature of the THz magnetic response,
so far, has been reported due to the lack of sufficient intense THz light sources. It is necessary to explore more
efficient excitation method to drive the antiferromagnetic spin system into the nonlinear regime. In this study wehave established a novel THz magnetic excitation method based on the field enhancement with the split ring
resonator (SRR) [2], and have clarified the nonlinear feature of antiferromagnetic spin dynamics by using thismagnetic excitation method. The SRRs array was deposited on the HoFeO3 crystal surface to generate the strong
THz magnetic field. We measured time-development of the local magnetization change induced by strong THz
pulse excitation in the vicinity of the SRR by using the time-resolved magneto-optical (MO) microscopy.

0.4 (a)
0.2
0.0
-0.2
-0.4 300 K

HoFeO3100%

0.575 (b)
0.570
0.565
100%
10%

0.560
0.555
1.0 (c)
0.8
0.6
0.4
0.2
0.0
0
10

20
30
Time (ps)40

50

Fig. 1: (a) Time response of the magnetization changeon the HoFeO3 crystal surface induced by THz
magnetic field. (b) Instantaneous frequency and (c)envelope amplitudes for the different excitation
intensities.

Classifying neuronal spike trains as analog or digital coding
Nonlinear dynamics group, Yasuhiro Mochizuki
Abstract Sensation and motion are represented and processed in the brain as series of neuronal
discharges called spikes or firings. Here we classify in vivo spike trains either as analog or digitalcoding, and compare information representation strategy among different brain regions.
© 2015 Department of Physics, Kyoto University

60

Ornstein-Uhlenbeck Process (OUP)
Rate [Hz]Rate [Hz]Animal’s behavior is operated by neuronal spikes in the brain. A central issue of neural coding is to discern themanner in which the sensory information is represented and processed, and finally transformed into motion, all interms of neuronal spikes.
Assuming that neural systems have evolved to optimize their efficiency in information transmission andprocessing, it is expected that individual areas of the brain process signals in different ways depending on theirfunctions. For instance, the elaborate control of motion may require delivering continuous values. In such a case,
it is expected that continuous values are transmitted in terms of the neuronal firing rates [1]. On the contrary, there
may be other kinds of computation that needs to transmit and process discrete signals as in decision processes.
Such information processing can be manifested by jumping among quasistationary attractor states of neuralnetworks represented as sets of distinct firing rates [2,3]. It has also been shown theoretically that the structure of
the tuning curve that achieves the optimal information transmission rate can be either continuous or discrete
depending on the length of the coding window [4,5]. Therefore, some brain regions may transmit continuous
values by modulating continuous firing rates, whereas other regions may process information in terms of discretevalues by switching discrete firing rates.
Here we suggest classifying spike trains according to whether the underlying firing rate of a spike train
is likely to be continuously varying or switching between active and inactive states [6]. In this contribution, weapply this method to in vivo spike trains, and report the results of the analysis.

30
0

0

5

10

60

0

Spike train
5

10

30
0

60

0

5

10

Empirical Bayes Model (EBM)

30
0

0

Hidden Markov Model (HMM)
Rate [Hz]

60

0

0

5

10

5

10

Spike train

Rate [Hz]Rate [Hz]Rate [Hz]0

Switching State Process (SSP)30

5Time [s]1060

Hidden Markov Model (HMM)

30
0

60

0

5

10

Empirical Bayes Model (EBM)

30
0

0

5
Time [s]10Fig. 1. Analog-digital classification of neuronal spike trains.

References[1] A. P. Georgopoulos et al., J. Neurosci., 2, 1527-1537 (1982).
[2] D. J. Amit, “Modeling Brain Function: The World of Attractor Neural Networks”, (Cambridge University
Press, Cambridge, 1989).
[3] M. Abeles et al., Natl. Acad. Sci. USA, 92, 8616-8620 (1995).
[4] M. Bethge et al., Phys. Rev. Lett., 90, 088104 (2003).
[5] A. Nikitin et al., Phys. Rev. Lett., 103,138101 (2009).
[6] Y. Mochizuki and S. Shinomoto, Phys. Rev. E, 89, 022705 (2014).

Quantum Gas Microscope of Two-electron Atoms– high-resolution imaging of ultracold ytterbium atoms in a 2D
optical lattice –Quantum Optics GroupRyuta Yamamoto

Abstract We develop an ytterbium (Yb) quantum gas microscope (QGM). We successfully detect single
174
Yb atoms in a two-dimensional optical lattice. The achieved resolution of 212nm is less than the lattice
constant of 266nm. This result opens up various possibilities of a QGM using fermionic and/or bosonicYb atoms.
© 2014 Department of Physics, Kyoto University

Ultracold atom system in an optical lattice is a quite useful platform for studying the condensed matter physics.
In recent studies, it is reported that a single atom in an optical lattice is observed with a high-resolution imaging
system [1,2]. This detection method is called a “Quantum Gas Microscope (QGM)”, by which we can directly
observe local properties and quantum dynamics with a single site resolution in an optical lattice [2,3,4]. In spite ofits great importance, a QGM is realized only for a bosonic rubidium (Rb) atom. From the viewpoint of quantum
simulation of condensed matter system, it is strongly desired to realize a QGM of a fermionic atom.
In this work, we successfully develop a QGM using an ytterbium (Yb) atom, which is an alkaline-earth likeatom and has many stable isotopes of 5 bosons and 2 fermions. We have already reported that we succeeded incooling and detecting a bosonic isotope of 174Yb in a two-dimensional (2D) optical lattice with “dual molasses”
technique in which we use two kinds of molasses beams; one is used as a probe (“blue molasses”) and another one
is used as a cooling (“green molasses”) [5]. Quite recently we improve the imaging system, such as the focus
positon and the tilt of objective lens, for achieving a higher resolution. We also enhance the cooling efficiency of“dual molasses” by applying a sideband cooling [6] in addition to the Doppler cooling. As a result we succeed in
keeping the fluorescence in a quite long time and almost successfully freeze atoms in the same lattice site.
Moreover, we combine the improved system with the high resolution spectroscopy of the ultranarrow opticaltransition between the ground 1S0 state and the metastable 3P2 state. As a result, we successfully prepare several
tens of atoms and detect only a few isolated atoms in a 2D optical lattice with high sensitivity, as shown in Fig.
1(a). We average over fluorescence signals of 10 single isolated atoms and the resolution of average image is212(4) nm, which is 1.89(3) times of the ideal
resolution, as shown in Fig.1 (b).
Further tuning of experimental conditions of
our QGM will enable us to perform many
interesting quantum simulation studies using Yb
atoms.

Fig. 1. (a) Fluorescence image of dilute cloud of atoms in a 2D
optical lattice. (b) The fluorescence distribution of single
isolated atom by averaging over 10 single atoms. The line is afit with the Gaussian. It shows the resolution σ (1/e1/2
half-length) of imaging system is 212(4) nm, which is 1.89(3)times of the ideal resolution.

References[1] W. S. Bakr et al. Nature 462, 5 (2009)[2] J. F. Sherson et al., Nature 467, 68 (2010).
[3] W. S. Bakr et al., Science 329, 547 (2010).
[4] T. Fukuhara et al., Nature Physics 9, 235-241(2013).
[5] K. Shibata et al., J. Phys. Soc. Jpn. 83,014301 (2013).
[6] D. J. Wineland et al., Phys. Rev. A 20, 1521
(1979).